The present invention relates to an improvement in aqueous processes for extracting bitumen from tar sands. This invention particularly relates to treatment of bitumen-containing sludge formed in waste water storage ponds associated with the hot water extraction of bitumen from tar sands. More particularly, this invention relates to a method for storing the sludge layer formed in a waste water retention pond to provide sludge storage zones substantially free of surface contamination.
Tar sands, which are also known as oil sands and bituminous sands, are siliceous materials which are impregnated with a heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels--just equal to the world-wide reserves of conventional oil, sixty percent of which is in the Middle East. By way of contrast, the American Petroleum Institute estimated total United States oil reserves at the end of 1965 at 39.4 billion barrels.
The tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 percent to 21 percent by weight, with a typical content of 13 weight percent of the sand. The oil is quite viscous--6.degree. to 8.degree. API gravity--and contains typically 4.5 percent sulfur and 38 percent aromatics.
The sands contain, in addition to the oil and sand components, clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30 percent. The sands also contain a small amount of water, in quantities of 1 to 10 percent by weight, in the form of a film around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. In the so called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface active agent added, or a solution of a neutral salt in water, which salt is capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, as disclosed in Canadian Pat. No. 841,581 issued May 12, 1970, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree.F., and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree.F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and some oil is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover an additional amount of oil therefrom.
The middlings layer, either as it is recovered from the primary process or as it is recovered after the scavenger step, comprises water, clay, and oil. The oil content is, of course, higher in middlings which have not undergone secondary scavenger steps.
Hereinafter in this specification, the term "effluent discharge" will be used to describe middlings material of depleted oil content which has undergone final treatment and which comprises some bitumen and clay dispersed in water. Also, the term "effluent discharge" includes the sand tailings of either a primary extraction cell or a scavenger cell, each of which contains some clay and bitumen. In essence, the term "elffluent discharge" includes all waste water streams associated with aqueous extraction of bitumen from tar sands.
In a hot water extraction process, effluent discharge, which is the combined waste water streams from the entire extraction process, is removed from the process plant as a slurry of about 35 to 55 and typically 45 percent solids by weight. Included in the slurry are sand, silt, clay, and small quantities of bitumen. In this specification, sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh but is larger than 2 microns. Clay is material smaller than 2 microns including some siliceous material of that size. Included, also, in the slurry are quantities of bitumen ranging from about 0.5 to 2.0 weight percent of the total discharge.
Because the effluent contains oil emulsions, finely dispersed clay with poor settling characteristics, and other contaminants, water pollution considerations prohibit discarding the effluent into rivers, lakes, or other natural bodies of water. The disposal of the effluent discharge has, therefore, presented a problem. Currently, effluent discharge is stored in retention ponds which involve large space requirements and the construction of expensive enclosure dikes. A portion of the water in the retention pond is recycled back into the hot water extraction process as an economic measure to conserve water storage space. However, experience has shown that the dispersed silt and clay content of the recycled water can reduce primary froth yield by increasing the viscosity of the middlings layer and retarding the upward settling of oil flecks. When this occurs, the smaller oil flecks and those that are more heavily laden with mineral matter stay suspended in the water of the separation cell and are removed from the cell with the middlings layer.
Effluent discharge from the hot water process for extracting bitumen from tar sands are generally disclosed contains a substantial amount of mineral matter much of which is colloidally dispersed in the effluent discharge and, therefore, does not settle very readily when stored in the retention pond. The lower layer of the retention pond can contain at least 5 percent and up to 50 percent of dispersed clay and silt as well as up to 5 percent bitumen. This part of the pond water is normally referred to as sludge. Sludge is not suitable for recycling to the hot water extraction process for the reason that its addition into the separation cell or the scavenger cell at the normal inlet means would raise the mineral content of the middlings of the cells to the extent that recovery of bitumen would be substantially reduced.
Generally, the settling which does take place in a storage pond provides a body of water in which the concentration of mineral matter increases substantially from the surface of the pond to the bottom thereof. As a typical example, a pond of effluent discharge having a surface area of about 1000 acres and an average depth of 40 feet can be characterized somewhat as follows:
a. From the surface of the pond to a depth of 15 feet, the mineral concentration which is primarily clay is found to be about 0.5 up to 5.0 weight percent. This pond water can normally be recycled to a hot water extraction process without interfering with the extraction of bitumen from tar sands. Also, the surface of the pond contains bitumen and liquid hydrocarbon diluent which float to the surface during the time of storage. A pond with liquid hydrocarbons floating on the surface can be damaging to wildlife, particularly wild ducks and geese if they land on the pond.
b. The layer of water in the pond between 15 and 25 feet from the surface contains between 5.0 to up to 15 percent mineral matter and some bitumen. This water, if recycled to the separation cell feed with fresh tar sands, would increase the mineral content of the middlings portion of the cell to the point that little bitumen would be recovered.
c. Finally, the section of the pond between 25 feet and the bottom of the pond contains 15 percent up to 50 percent mineral matter.
In general, pond water containing more than 5 weight percent mineral matter can be referred to as sludge. Thus, water in areas of the pond described as (b) and (c) above can be included in the general definition of sludge in the present description.
Although some settling in the sludge layer is realized, the rate is such that over a period of years it is hardly noticeable. However, when the aqueous sludge is disturbed, the balance is upset so that often the bitumen in the sludge is loosened from the minerals to which it is attached and floats to the surface of the pond. This is particularly noticeable if sludge is pumped from one storage zone to another. Sludge is sometimes transferred from a pond to make more room for sand and other solids.
Bitumen on the surface of any outdoor storage pond can be detrimental to wildlife in the area, particularly birds, if they land upon or swim over the surface of the pond. Therefore, it is desirable to store sludge in a manner by which the quantity of bitumen on the surface of the storage area is minimized. Thus, it is desirable to store sludge in an outdoor area so that the upper layer of the storage zone contains little or no bitumen and mineral matter. By the methods of the present invention, this object can be accomplished.